A solid state image pickup device is constructed of a plurality of pixels of image pickup elements, each having a photosensitive section. In each image pickup element, light incident to a light receiving section is converted into electric charge signal by the photosensitive section (e.g., a photodiode). This electric charge signal is outputted from a charge transfer section of the solid state image pickup device.
As a method of improving the light reception sensitivity of such a solid state image pickup device without enlarging any light receiving section, a light converging lens is mounted above a photodiode (photosensitive section) to converge external light onto the photosensitive section.
A conventional method of manufacturing a solid state image pickup device having a light converging lens will be described with reference to FIGS. 13A to 13D.
As seen from FIG. 13A, a plurality of photosensitive sections (photodiodes) 2 are first formed on the surface of a semiconductor substrate 1 at predetermined areas. The surface of each photodiode 2 is covered with a passivation film 3. Color filters 4.sub.1, 4.sub.2 and 4.sub.3 are formed on the surface of the passivation film 3, using a dying film such as gelatin. The color filters 4.sub.1, 4.sub.2 and 4.sub.3 are red, green, and blue, respectively. A passivation film 5 is deposited on the surface of these color filters 4.sub.1, 4.sub.2 and 4.sub.3.
Next, as seen from FIG. 13B, a photoresist film 6A is formed over the whole surface of the passivation film 5. The photoresist film 6A is patterned to form a photoresist film 6 above each photodiode 2.
Thereafter, as shown in FIG. 13C, light 7 is applied to the photoresist films 6 to make them transparent.
Then, as shown in FIG. 13D, the transparent films 6 are thermally deformed to form light converging lenses 6'.
FIGS. 11A to 11C show a conventional solid state image pickup device manufactured by the above-described method. FIG. 11C is a plan view, FIG. 11A is a cross sectional view taken along the A.sub.5 -A.sub.5' of FIG. 11C, and FIG. 11B is a cross sectional view taken along line B.sub.5 -B.sub.5' of FIG. 11C. FIG. 11A corresponds to FIG. 13D.
Another conventional manufacturing method will be described with reference to FIGS. 14A to 14D.
The processes up to forming a passivation film 5 shown in FIG. 11A are the same as those described with FIG. 13A. A transparent film 106 to form light converging lenses 106' (refer to FIG. 14D) is formed on the passivation film 5.
Next, as seen from FIG. 14B, a photoresist layer 107A is formed on the transparent film 106. The photoresist layer 107A is patterned by means of a photoetching method, to form a photoresist film 107 patterned so as to form light converging lenses. Thereafter, as seen from FIG. 14C and 14D, the transparent film 106 and photoresist film 107 are etched by means of an anisotropic etching (e.g., RIE). The shape of the photoresist film 107 is therefore transferred to the transparent film 106 to form the light converging lenses 106'. The plan view and cross sectional view along line B.sub.5 -B.sub.5', of this device are shown in FIGS. 11C and 11B.
As shown in FIG. 14B, the photoresist film 107 of a solid state image pickup device manufactured by the method explained with FIGS. 14A to 14D is formed on a flat passivation film 5. Therefore, the film thickness is substantially the same both in the X- and Y-directions. Thus, the curvatures in the X- and Y-directions are determined by the shape of the photoresist film 107 patterned so as to form the light converging lenses. The light convergence efficiency in the direction along line A.sub.5 -A.sub.5' of the light converging lens 6' (106') shown in FIG. 11A is good because light is refracted by the light converging lens 6' and directed to the center of the photodiode 2. However, the light convergence efficiency in the direction along line B.sub.5 -B.sub.5' is not so good because of a so-called "circular aberration" phenomenon caused by a larger curvature at the plane along line B.sub.5 -B.sub.5' of the light converging lens 6' (106') than that along line A--A. Specifically, as seen from FIG. 12 showing the lens effects, light in the Y-direction (B--B direction) can be sufficiently converged, but light in the X-direction (A--A direction) cannot be sufficiently converged. Therefore, an effective light convergence area 9 becomes small. In other words, the light convergence of the light converging lens 6 becomes small.
If the curvature is set so that light in the Y-direction can be sufficiently converged, the length of each pixel becomes greater in the X-direction than in the Y-direction. Therefore, contrary to the above-described case, light in the X-direction cannot be sufficiently converged. Most of light near each photodiode accordingly cannot be used. This is serious particularly for a PAL method having more pixels in the vertical direction than the horizontal direction.
As described above, a conventional device cannot obtain a sufficiently large effective light convergence area 9 (FIG. 12). Therefore, if the ratio of an X-direction width to a Y-direction width of the micro light converging lens 6' (106') changes, the light converging lens effects in the Y-direction may disappear in some cases. Namely, both opposite areas in the Y-direction of the photodiode 2 may become an invalid light convergence area 10. As a result, light incident on the invalid light convergence area 10 may enter adjacent photodiodes, resulting in a problem of increased color crosstalk, smear, and the like.
It is conceivable that the curvature is made small by thinning the film thickness of the light converging lens 6' (106') at the cross section in the Y-direction or along line B.sub.5 -B.sub.5'. However, this essentially makes small the curvature of the light converging lens at the cross section in the X-direction or along line A.sub.5 -A.sub.5'. The effective light convergence area therefore becomes small. If the length in the X-direction of the photodiode is made large to compensate for the reduced effective light convergence area, the problem of increased smear will occur.